Networks in accordance with the IEEE 802.11 standards are also dubbed Wi-Fi networks. They are used to network stations (for example computers, personal assistants and peripherals) in numerous applications.
The 802.11 standard defines in the document “IEEE 802.11a-1999, IEEE 802.11b-1999, IEEE-802.11d-2001, Part 11: wireless LAN medium access control (MAC) and physical layer (PHY) specifications” a method for regulating the traffic in the wireless network. This method uses a system of congestion windows (Congestion Window “CW”) to regulate this traffic. According to this standard, to determine the instant at which to send a data packet, a station draws a random number between 0 and CW-1, the value CW being an integer lying between two values CWmin and CWmax specified by the 802.11 standard.
This value CW serves as backoff counter for the sending of the packet, this counter being deferred if the station notes that another station is in the process of sending. Unfortunately, this system based on congestion windows generates a significant number of collisions on the wireless network, this being manifested, from the user's point of view, by a significant loss of bandwidth.
Another mechanism known by the name “tournament scheme” may be used by the various stations to regulate the sending of packets and to limit the collision rate. This tournament scheme is described in the document by the authors Z. Abichar and M. Chang, entitled “CONTI: Constant Time Contention Resolution for WLAN Access”, IFIP Networking 2005.
The tournament scheme consists in organizing a sort of tournament between the stations having to send a packet. A tournament is composed of a certain number of selection rounds, each executed for a time interval of predefined duration allowing each station to have time to hear the sending of any other. This duration can typically be equal to the duration of the time interval dubbed “SlotTime” which, according to the IEEE 802.11 norm, is defined as elementary interval in the congestion window based contention resolution procedure.
At the start of the tournament, all the stations having to send a packet may be authorized to send this packet. At each selection round, one or more stations may be deleted from the list of the stations authorized to send, depending on whether an authorization or a prohibition to send is allotted to them during this selection round. On completion of the tournament, only the non-eliminated stations are authorized to send. If several stations remain in the running on completion of the tournament, they send at the same time, thus causing a collision and therefore disturbed reception with no possibility of correctly receiving the data packets sent. These stations will then have to participate in the next tournament so as to attempt to send these packets again.
In the case of the sending of data streams requiring priority access to the network or a high bandwidth, the need is apparent to provide a solution for regulating sending make it possible to guarantee a performance level in terms of bandwidth or of bitrate and also a quality of service.
Patent application WO 2009/095628 describes a solution, using the tournament scheme and allowing, when in the presence of stations having streams of various priority levels to be sent, equitable sharing of access to the radio medium between the various stations.
In this solution, the values representative of an authorization or prohibition to send which are assigned to a station for the selection rounds of a current tournament are deduced from an index value, assigned to the station for the current tournament and calculated on the basis of an index value assigned to the station for an earlier tournament.
Patent application WO 2009/095628 more precisely describes a scheduling scheme of Round Robin type, designed to be implemented between stations having to send packets and belonging to one and the same class of packets. By listening to the signals sent during the selection rounds of a tournament, a station can determine the series of values representing the authorizations or prohibitions obtained in the course of the various selection rounds by the station winning the tournament. This series of values allows the station to determine, on the basis of the index value A used by this station for the determination of the values representing the authorizations or prohibitions which are allocated to it in the course of the current tournament, an index value A′, to be used for the determination of the values representing the authorizations or prohibitions of the various selection rounds to be used for the following tournament.
The value A′ is in particular calculated on the basis of the value A, by a circular permutation operation of length Ai on the value of A in the interval [0; 1[, which is defined as follows:A′=A+1−Ai if A<Ai A′=A−Ai if A≧Ai 
where [Ai−1, Ai[ is the value interval containing the index value Awin used by the station that has won the current tournament, with i=1 to Z (Z being a positive integer). Note that, as described in patent application WO 2009/095628, the Z intervals [A0, A1[, . . . [Ai, Ai+1[, . . . [AZ−1, AZ[ form a partition of the interval [0; 1[ with A0=0 and AZ=1.
The solution described in patent application WO 2009/095628 does not operate in an optimal way when the signals sent by the stations during the selection rounds are scrambled, because for example of propagation problems related to obstacles or of poor radio conditions.
Consequently, the value of Ai cannot be determined correctly by all the stations implementing the Round Robin scheme and the circular permutation performed on the index value A by one station will not be identical to the circular permutation performed by the other stations. The performance of the Round Robin is affected thereby in that equity of access to the radio medium is no longer guaranteed. In particular, the number of collisions between stations may therefore increase appreciably, giving rise to a degradation of the transmissions.